I just give them as a way of saying happy birthday or good job or nice to meet you. If someone gives me a llama badge I always give one back but llamas are pointless so I don't care if I get one back or not.
Well I always give llamas to people who favorite my stuff. Seeing as most people don't like 'thanks for the watch/fav/whatever' comments i usually just send them instead, it seems better than just doing nothing. I like to let them know I appreciate it without spamming their page. As for if i get them back, not usually, but I guess it isn't too much of a big deal